Former Scotland Office director Alun Evans in full fiscal autonomy call

Mr Evans was involved in the negotiations over the referendum-enabling Edinburgh Agreement, in which the UK Government pointedly rejected Alex Salmond's plan for a second ballot question on "devo max".

Current Scottish Secretary David Mundell has said full fiscal autonomy would leave a "disastrous" £10 billion hole in Scotland's finances.

With the SNP now occupying all but three of Westminster's 59 Scottish seats and about to set the terms for another referendum, Mr Evans said only full devolution of tax and spend, except for reserved areas, will avert a vote for independence in another referendum.

In a lecture to the British Academy in London, where he is now chief executive, Mr Evans said: "Since the referendum, the SNP has still been calling all the shots.

"The Yes campaign - which lost the referendum by over 10% of the votes - has maintained the initiative. The referendum was meant to settle the issue for a generation. It hasn't done so even for a year."

He added: "The time for incrementalism is over. Playing catch up with the SNP has not worked and probably will not work.

"The time is ripe now to get ahead of the curve and so help to secure the union. I would argue that the time has come for the United Kingdom to make a big, bold, and generous offer to the people of Scotland.

"That offer needs to be - whatever people choose to call it - full fiscal autonomy, devo max plus or, in the language of Gladstone, home rule for Scotland within the United Kingdom."

Scotland should have full responsibility for taxation, spending, and domestic policy including offshore energy, with monetary policy set by the UK central bank and defence and foreign policy run by Westminster in consultation with Holyrood, he said.

This arrangement would spell the end of the Barnett formula - the "outdated" system for calculating Scotland's budget - with Scotland paying Westminster for shared UK services, he suggested.

This agreement should come with a cut in Scottish MPs "of perhaps 50%" and stipulate that it is for the long term to prevent it being "immediately unpicked", he added.

Mr Evans's view of "home rule" is similar to the SNP's full fiscal responsibility plan - making Holyrood responsible for raising, collecting and administering all, or the vast majority of, revenues and spending with a remittance to the United Kingdom to cover common UK public goods and services, such as defence and foreign affairs.

The SNP has more recently called for the Barnett formula to remain unless and until a transition to full fiscal autonomy is completed.

Mr Evans's call for a reduction in the number of MPs could prove problematic for nationalists, as it would reduce Scotland's representation on key reserved votes such as the renewal of Trident.

In his speech, Mr Evans said the Scottish Parliament is popular, far more trusted and arguably more effective than the pre-devolution system.

Holyrood has latterly been governed by "a remarkable triumvirate of politicians" comprising the "effective" Alex Salmond, his successor Nicola Sturgeon and her "very competent" deputy John Swinney, he argued.

He contrasted this with the "dearth of unionist politicians wishing to take on, or capable of taking on, the SNP" - namechecking two of his former Scotland Office political bosses, Michael Moore and Alistair Carmichael.

Westminster's defence of the United Kingdom was "the electoral equivalent of trench warfare", winning ground one day before losing it the next until the two camps were almost neck and neck.

Its "vow" of more powers was seen as a last-minute "panic reaction" to an SNP-led campaign which "appeared to be calling most of, if not all of, the shots".

On the anniversary of the publication of "the vow", Mr Evans said it was delivered amid "an air of panic" with the Prime Minister cancelling Westminster proceedings, the Queen urging voters "to think very carefully" and US President Baracl Obama calling for the UK to remain "robust and united".

He suggested Mr Cameron's victory pledge to deliver swift Scottish devolution "critically coupled with a statement in support English Votes for English Laws" may have been "a sign of continuing panic".

He said: "Is the inevitable outcome that UK government, of whatever party, will always be playing catch-up with the SNP?

"Will there be a second referendum sooner rather than later - as Nicola Sturgeon is already putting forward as an issue for the 2016 Holyrood election?

"And will the outcome for that referendum be a Yes vote - as suggested by some recent opinion polls?

"I don't know but I don't think it has to be that way.

"For unionist politicians and those who believe in the future of the United Kingdom, doing nothing is not an option."